Microstructures and dislocations in the stressed AZ91D magnesium alloys

Transcription

1 Materials Science and Engineering A344 (2002) 279/287 Microstructures and dislocations in the stressed AZ91D magnesium alloys R.M. Wang a,b,, A. Eliezer a, E. Gutman a a Ben-Gurion University of the Negev, PO Box 853, Beer-Sheva 84105, Israel b Electron Microscopy Laboratory and State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing , People s Republic of China Received 3 August 2001; received in revised form 4 June 2002 Abstract The microstructures and the dislocation arrangements in the die cast AZ91D magnesium alloy as well as the stressed alloys have been investigated using transmission electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray (EDX) analysis. Besides the dominant a-mg and b-mg 17 Al 12 phases, Al 8 Mn 5 and Mg 5 Al phases have also been found and studied in the alloy. Dislocation pile-ups have been found in the stressed AZ91D alloys for the first time. They are confined in the slip planes and piled up against the grain boundaries. The dislocation pile-ups increase with deformation till about 2.1% and then remain almost identical even deformed to higher degree. The dislocations pile-ups in the AZ91D magnesium alloy are found to be beneficial to the resistance to stress corrosion cracking of the alloy, and thereby beneficial to the mechanical properties of the alloy. Dislocation networks are found to increase with deformation in all cases. The dislocation networks have also been found in the b- Mg 17 Al 12 phase as well as in the matrix in the deformed AZ91D magnesium alloys. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Dislocation; Microstructure; Magnesium Alloy; Mechanical property 1. Introduction The attractive mechanical properties of magnesium and its alloys increase their use for many technical applications [1,2], i.e. automobile, aeronautical and aerospace [3], housing utensils [4], electronic industries, etc. However its poor galvanic corrosion resistance has been a major obstacle to its growth in structural application despite of its other desirable physical properties. Moreover, the problem of stress corrosion is becoming a major one today in Mg-alloys [5]. The susceptibility to intergranular stress corrosion cracking of magnesium base alloys depends strongly on alloy composition and heat treatment, and thus on the microstructures of the alloys. However, the fundamental factors that render an alloy susceptible are still far from being understood. A lot of work has been done in Corresponding author. Tel.: / ; fax: / address: rmwang@pku.edu.cn (R.M. Wang). aluminum alloys and dislocation pile-ups against the grain boundaries in the bands have been found and used to predict the susceptibility to stress corrosion cracking [6]. In this paper, the microstructures of the AZ91D magnesium alloy and the dislocation arrangements in the stressed states were studied using conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and EDX analysis. Dislocation pile-ups have been found for the first time in the alloy. 2. Experimental procedures The specimens of AZ91D alloys used for this study were die cast in a 200-t cold chamber machine. Round (gauge length of 75 and diameter of 5.9 mm) specimens were produced at casting temperature of 650 8C, injection rate of 57 m s 1 and die casting temperature /02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S ( 0 2 )

2 280 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/287 Table 1 Chemical composition of the investigated AZ91D magnesium alloy (mass.%) Elements Al Mn Zn Si Ni Cu Fe Be Mg Chemical composition rest Fig. 1. Schematic diagram of the AZ91D specimen for mechanical testing. of 200 8C. Their chemical compositions are given in Table 1. Fig. 1 shows the schematic diagram of the AZ91D specimen for mechanical testing. The thickness of the testing specimen is 4 mm. The tested mechanical properties of the AZ91D alloys stressed up to specified plastic strain are summarized in Table 2. Specimens for TEM and HRTEM analyses were prepared as follows. First, slices with dimensions 10 / 4 /0.2 mm were cut from the center of the stressed AZ91D specimens where the stress is concentrated. Then they were mechanical polished and cut into F3 mm disks, and dimpled to about 10 mm in the center. Final preparation of the specimens was ion milling on a Gatan precision ion polishing system 691 under conditions of 5.0 kv and incident angle of 6/48. TEM analysis was conducted on a JEOL 2010 transmission electron microscopy equipped with a Gatan Multiscan CCD digital camera. The software for the CCD camera is Digital Micrograph EDX spectroscopy was performed using an Oxford Link ISIS 6498 spectroscopy attached to JEOL 2010 transmission electron microscopy. The software for the chemical composition calculation was LINK ISIS version Results and discussions 3.1. Microstructure of the die cast alloy The microstructure of the AZ91D magnesium alloy consists of a bimodal distribution of a-mg (Hexagonal crystal structure, Space group P6 3 /mmc, a/ and c / nm) grains together with intergranular b-mg 17 Al 12 (Cubic crystal structure, Space group I/ 4/3 m, a/1.056 nm) grains. Fig. 2(a) shows a typical bright field image of the AZ91D magnesium alloy showing the distribution and morphology of the dominant a-mg and b-mg 17 Al 12 phases. A great deal of TEM observation indicates both a-mg and b-mg 17 Al 12 particles have a wide variety of morphologies. The sizes of the b- Mg 17 Al 12 grains are measured to be mostly about several micrometers. Fig. 2(b) shows a high-resolution image of the b-mg 17 Al 12 phase and the matrix. Selected area diffraction (SAD) analysis indicates that the incident beam is parallel to [111] b. The interplannar spacings of (1/ 1/0) and (10/ 1) planes in the b-mg 17 Al 12 phase are calculated to be nm. In Fig. 2(b), typical trigonal crystal structure of b-mg 17 Al 12 phase along [111] zone axes can be clearly seen. The interplannar spacings of (1/ 1/0) and (10/ 1) planes of the b- Mg 17 Al 12 phase in Fig. 2(b) are measured to be about 0.75 nm, which is consistent with the calculated result. It can also be found that both (1/ 1/0) and (10/ 1) planes consist of three very fine layers with interplannar spacings about 0.25 nm. It can also been seen that there exist some parallel stripes in the right of Fig. 2(b). SAD and HREM analyses reveal that the stripes in Fig. 2(b) correspond to (1/ 1/00) planes of the a-mg matrix. The interplannar spacings of the stripes are measured to be about 0.28 nm, while the calculated spacing between each (1/ 1/00) planes of a-mg is nm. Amorphous layer with about 1/2 nm in widths have also be found along the interface between the two phases in Fig. 2(b). Such kind of amorphous layer along the interface is beneficial to transfer stress between the two phases. Therefore, it is good to the mechanical properties of the alloy. Besides the dominant a-mg and b-mg 17 Al 12 phases in the AZ91D alloy, some trace precipitates have also been found in the alloy. Fig. 3 shows a typical microstructure of the AZ91D alloy with such trace precipitates. Besides the well-known a, b phases, two kinds of phases also exist in the alloy. One kind of phase has a typical Table 2 Mechanical properties of the AZ91D alloys stressed up to specified plastic strains Samples TYS (MPa) TYS 0.2% (MPa) Stress max (MPa) Strain (%) State Z No rupture Z No rupture Z No rupture

3 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/ Fig. 2. Typical bright field image (a) and high-resolution image (b) of the bimodal distribution a-mg and b-mg 17 Al 12 phases in the die-cast AZ91D magnesium alloy. polygonal morphology with size about 100 /200 nm; the other one has a typical oval morphology with size about 50 nm. Energy dispersive X-ray (EDX) analyses have been performed on them. Fig. 3 shows the EDX spectrums of the two phases as well as those of the dominant a-mg and b-mg 17 Al 12 phases in the AZ91D alloy. The results indicate that the polygonal phase is rich in Al and Mn with a little amount of Mg, while the oval one is found to be mainly consisting of Mg and Al. The analyzed results are summarized in Table 3. The chemical composition of the matrix obtained from the Fig. 3. Typical microstructures (a) and corresponding energy dispersive X-ray spectrums (b) of the AZ91D magnesium alloy showing various kinds of precipitates. Table 3 Analyzed chemical composition of the phases shown in Fig. 3(a) (at.%) Phases Mg Al Mn Zn Matrix b-mg 17 Al Al 8 Mn Mg 5 Al

4 282 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/287 EDX is consistent with the nominal one. The Mg/Al ratio (68.15/29.63) of the b-mg 17 Al 12 phase is found to be a bit higher than the nominal ratio 17/12. The excessive Mg content in the spectrum may be contributed to the Magnesium matrix. For the polygonal phase, the Al/Mn ratio (57.01/35.14) is very close to 8/5. The trace amount of Mg is likely to be contributed to the matrix. Thereby, the phase is probably Al 8 Mn 5. However, for the oval phase, the Mg/Al ratio (82.23/16.43) is very close to 5. It is a new phase that has not been reported in literature. In this paper, it is written as Mg 5 Al. Probably it is a transient phase and may be the pre-formation of b-mg 17 Al 12 phase. SAD and high-resolution images analyses on the polygonal phase confirm the identification by EDX. Fig. 4(a) shows a typical bright field image of the Mnrich phase in AZ91D alloy. The typical polygonal morphology of the Mn-rich phase is clearly seen in Fig. 4(a). SAD pattern analyses indicate that the Mnrich phase has a hexagonal crystal structure with lattice parameters about a/1.27 and c /1.59 nm, which is consistent to the crystal structure and lattice parameters of the Al 8 Mn 5 phase (hexagonal crystal structure, a/ and c /1.588 nm). Fig. 4(b) shows a highresolution image of the Al 8 Mn 5 phase and the matrix. The incident beam is parallel to [/ 1/101] Al8Mn5. The interplannar spacings of (1/ 1/01) and (01/ 1 1) planes in the Al 8 Mn 5 phase in Fig. 4(b) are measured to be about 0.9 nm, which is very close to the calculated nm. In the right of Fig. 4(b) is the a-mg matrix. The interplannar spacing between the (1/ 1/00) planes is nm. Different from that between a/b phases, the interface between the Al 8 Mn 5 phase and the matrix is straight, sharp and smooth, which is not in favor of transferring stress between the hard and brittle Al 8 Mn 5 phase and the ambient Mg matrix. Although without deformation, the dislocation density in the a-mg matrix tends to be rather high, which is no doubt due to the fast cooling under geometric constraint that was experienced during die casting period. Fig. 5 shows a bright field image and corresponding dark field image of the dislocation arrangements in the AZ91D magnesium alloy. A lot of dislocations are arranged in networks indication that considerable recovery has also occurred during cooling to room temperature. The average dislocation spacing is measured to be about 100 nm ranging from about 10 nm to about 1 mm Dislocations in the stressed AZ91D alloys Fig. 6 shows typical TEM images of the dislocation arrangements in the AZ91D alloy after 0.43% deformation. The dislocation arrangements in Fig. 6(a) correspond to those formed during cooling to room temperature as shown in Fig. 5. After tensile test, the Fig. 4. Typical bright field image (a) and corresponding high-resolution image (b) of the Al 8 Mn 5 phase in the AZ91D magnesium alloy. dislocation density increases apparently. The dislocation arrangements in the networks become more complex. The average dislocation spacing in the network is measured to be about 50/100 nm, which is less than that in the die cast AZ91D alloy, indicating higher dislocation density. Moreover, besides the dislocation networks formed during the cooling period, series of straight and parallel dislocations have also been found to exist in the deformed AZ91D magnesium alloy. Fig. 6(b) shows this kind of dislocation arrangements. A

5 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/ Fig. 5. Bright field image (a) and dark field image (b) of the dislocation arrangements in the AZ91D magnesium alloy. series of straight and parallel dislocations are confined in the slip plane and piled up against the grain boundary. The spacings between these parallel dislocations are measured to be about 50/200 nm. This kind of dislocation arrangements has been found frequently. However, in some cases, something like superlattice pileups has also been found, as shown in Fig. 6(c). Each dislocation superlattice consists of several very short dislocations of about 10 /20 nm in length and each superlattice pile-ups consists of several such dislocation superlattices with spacing about 50 nm. After higher deformation test, the density of the dislocations pile-ups in the alloys increases significantly. Fig. 7(a) shows typical dislocation arrangements in the alloy. Fig. 7(b) is a high magnification image of Fig. 7(a). A lot of dislocation pile-ups can be found in Fig. 7(a). The dislocation pile-ups are confined in various slip planes. The spacing between each parallel dislocation in the pile-ups is measured to be only several nanometers, which is much smaller than that in the AZ91D magnesium alloy after 0.43% deformation. The lengths of the dislocations in the pile-ups are found to about tens of nanometers. The spacings between the pile-ups are measured to be about 100/200 nm. In a word, the densities of the dislocation networks and pile-ups in the investigated AZ91D magnesium alloy after 2.1% deformation are apparently higher than those after 0.43% deformation. Speidel et al. have studied the relationship between the susceptibility to stress corrosion cracking and the dislocation pile-ups against the grain boundaries in the bands in aluminum alloys and found that dislocations pile-ups are resistant to stress corrosion cracking in aluminum alloys [6]. As shown in Table 2, the mechanical properties of the AZ91D magnesium alloy after 2.1% deformation are apparently higher than those after 0.43% deformation, which is probably contributed to the abundant amount of dislocation pile-ups. Dislocations have also been found in the b-mg 17 Al 12 particles in the AZ91D alloy after 2.1% deformation. Fig. 7(c) gives a bright field image of the b-mg 17 Al 12 particle in the alloy showing a great deal of dislocations. The dislocations in the b-mg 17 Al 12 phase form as networks and are confined within the phase. With even higher deformation test, the dislocation density increases significantly. Fig. 8a and b show typical bright field images of the dislocation networks and pile-ups in the alloy after 5.4% deformation. It can be found that the density of the dislocation networks in the alloy is the highest in the investigated alloys. Obviously it is because of the highest deformation. However, the distribution of the dislocation pile-ups in the alloy after 5.4% deformation remains almost similar to those after 2.1% deformation. The spacing between each parallel dislocation in the piles is found to be also about several nanometers. Compared with that of the AZ91D alloy after 2.1% deformation, the tensile yield strength of the AZ91D alloy after 5.4% deformation is only a bit higher. Perhaps it is because of similar density of dislocation pile-ups in the two states. Thereby, it can

6 284 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/287 Fig. 6. Typical bright TEM images of the dislocation arrangements in the AZ91D magnesium alloy after 0.43% deformation. (a) Dislocations formed during cooling to room temperature, (b) dislocation pile-ups, (c) dislocation superlattice pile-ups. be inferred that high-density dislocations pile-ups in the AZ91D magnesium alloy are beneficial to the resistance to stress corrosion cracking, and thus beneficial to the mechanical properties of the AZ91D magnesium alloy. In the AZ91D alloy after 5.4% deformation, a great amount of dislocations networks form in the b- Mg 17 Al 12 phase. Some dislocation pile-ups have also been found to against the a/b interface. Fig. 8(c) shows a bright field image of the dislocations arrangements in the b-mg 17 Al 12 phase. The dislocation networks are confined within the b-mg 17 Al 12 phase. The density is found to be the highest in the investigated alloys. At the bottom of Fig. 8(c), a series of dislocation pile-ups have

7 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/ Fig. 7. Dislocation arrangements in the AZ91D magnesium alloy after 2.1% deformation. (a, b) dislocation pile-ups, (c) dislocations in b-mg 17 Al 12 phase. also been found to end against the grain boundary between a-mg and b-mg 17 Al 12 phases. 4. Conclusions The microstructures of the die cast AZ91D alloy consist of dominant a-mg and b-mg 17 Al 12 as well as Al 8 Mn 5 and Mg 5 Al phases. The a-mg and b-mg 17 Al 12 particles in the AZ91D magnesium alloy have a wide variety of morphologies while Al 8 Mn 5 and Mg 5 Al particles have typical polygonal and oval morphologies, respectively. High-density dislocations in the a-mg matrix have been found in the die cast AZ91D magnesium alloy. The dislocation networks form during cooling to room temperature.

8 286 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/287 Fig. 8. Dislocation arrangements in the AZ91D alloy after 5.4% deformation. (a) dislocation network, (b) dislocation pile-ups, (c) dislocations in b- Mg 17 Al 12. Dislocation pile-ups have been found in the stressed AZ91D alloys. They are confined in the slip planes and piled up against the grain boundary. The density of the dislocation pile-ups in the alloy increases with deformation till about 2.1% and then remains almost identical even deformed to higher degree. High-density dislocations pile-ups in the AZ91D magnesium alloys are beneficial to the resistance to stress corrosion cracking. The density of dislocation networks in the die-cast AZ91D magnesium alloy increases with deformation. Dislocation networks have also been found in the b- Mg 17 Al 12 phase as well as in the matrix in deformed states.

9 R.M. Wang et al. / Materials Science and Engineering A344 (2002) 279/ References [1] D. Eliezer, E. Aghion, F.H. Froes, in: E. Aghion, D. Eliezer (Eds.), Proceedings of the First Israeli International Conference on Magnesium Science and Technology, Magnesium research Institute (MRI) Ltd., Beer-Sheva 84100, Israel, 1997, p [2] K. Harbodt, B.B. Clow, in: E. Aghion, D. Eliezer (Eds.), Proceeding of the Second Israeli International Conference on Magnesium Science and Technology, Magnesium Research Institute (MRI) Ltd., Beer-Sheva 84100, Israel, 2000, p [3] J.M. Arlhac, J.C. Chaize, in: G.W. Lorimer (Ed.), Proceedings of the Third International Magnesium Conference, the University Press Cambridge, UK, 1997, p [4] J. Abthoff, W. Gelse, J. Lang, in: G.W. Lorimer (Ed.), Proceedings of the Third International Magnesium Conference, The University Press Cambridge, UK, 1997, p [5] P.L. Bonora, M. Andrei, A. Eliezer, E. Gutman, in: E. Aghion, D. Eliezer (Eds.), Proceeding of the Second Israeli International Conference on Magnesium Science and Technology, Magnesium Research Institute (MRI) Ltd., Beer-Sheva 84100, Israel, 2000, p [6] M.O. Speidel, in: R.W. Staehle, A.J. Forty, D. Van Royen (Eds.), Proceeding of Conference Fundamental Aspects of Stress Corrosion Cracking, The Ohio State University, 1967, p. 561.